Various devices are known for amplifying electronic signals. Such devices are commonly divided into at least three basic groups known as class A amplifiers, class AB amplifiers, and class C amplifiers. Class A amplifiers may generally be described as linear, while class AB and class C amplifiers may generally be described as nonlinear.
Linear, or class A, amplifiers have an advantage of providing an output signal which varies predictably in a linear fashion from the input signal. Nonlinear amplifiers, in contrast, provide an amplification level which is not uniform over their input range. Because of their uniform gain characteristics, linear or class A amplifiers are preferred for many applications, e.g., certain types of transmitters or receivers in the field of communications (including television).
In addition, linear or class A amplifiers generally have favorable power efficiency with respect to amplification of AC signals, and are superior to nonlinear amplifiers, such as class AB and class C amplifiers, in this regard.
Despite some of the advantages of linear amplifiers, one drawback with using linear amplifiers is their relatively large DC power consumption, particularly when compared to the DC power consumption of nonlinear amplifiers such as class AB and class C nonlinear amplifiers. One reason for the relatively large DC power consumption is that class A amplifiers tend to draw significant DC power even in the absence of an AC input signal. In contrast, nonlinear amplifiers tend to draw no DC power or minimal DC power in the absence of an AC input signal.
In order to keep DC power consumption to a minimum, it would be desirable to use nonlinear amplifiers such as class AB and class C amplifiers for applications normally requiring linear amplification characteristics. To do so, it is often necessary to try to linearize the performance of the nonlinear amplifier, e.g., by utilizing a specific linear region of the operating range of a nonlinear amplifier.
For example, in one particular technique, a DC bias signal is used as a means to force the nonlinear amplifier to operate in a linear region of its operating range. An exemplary conventional system configured with a bias signal is shown in FIG. 12, which is a circuit diagram of an SD4010 silicon NPN planar transistor manufactured by SGS-Thomson Microelectronics and using diffused emitter ballast resistors. The FIG. 12 circuit includes a non-linear amplifier 1215 coupled to a bias signal 1210, a purpose of which is to attempt to linearize the performance of the non-linear amplifier 1215.
One drawback of using a DC bias signal 1210 such as shown in FIG. 12 is that determination of a suitable DC bias level ordinarily necessitates a tedious process of trial and error. Moreover, even after arriving at a suitable DC bias level, operation of the nonlinear amplifier is not made completely linear. Rather, variations in the amplitude of the input signal beyond a certain level will generally result in nonlinear operation. In addition, the energy of the bias signal in combination with the input signal power may cause burn out of the amplifier 1215.
In another conventional technique for linearizing non-linear amplifier operation, a class A pre-amplifier stage is used to boost the input signal level such that the input signal falls within the linear region of the nonlinear amplifier. The combination of class A and class AB amplifier, because of the nonlinear amplification stage, is commonly categorized as a class AB amplifier. A drawback of this technique is that it is necessary to tightly control the level of the output signal from the class A pre-amplifier, which may be difficult.
In addition to the difficulty in getting a nonlinear class AB or class C amplifier to behave in a linear fashion, nonlinear amplifiers in operation often suffer from the creation of undesired signal by-products. Third-order signal by-products (called intermods) are created when two or more input signals, or signal components at different frequencies of an input signal, are amplified. In non-linear amplifiers, the intermods grow in amplitude relative to the output signal as the input signal weakens. Thus, if the input signal is relatively low, then the undesired intermods may approach the same amplitude as the desired output signal. Linear amplifiers, in contrast, have third-order intermods that vary proportionately with the input signal. Thus, as the input signal weakens, the undesired intermods decrease in magnitude proportionately to the decrease in the output signal.
It would be advantageous to provide an amplifier having linear amplification characteristics resembling that of a linear amplifier, such as a class A amplifier, while at the same time having the DC power efficiency of a nonlinear amplifier, such as a class AB or class C amplifier. It would further be advantageous to provide such an amplifier capable of rejecting or minimizing the effect of undesired spectral by-products such as intermods.